The technique disclosed in Patent Literature 1 has been known with regard to this type of open-close device for fuel tank. The open-close device for fuel tank includes: a tank opening-forming member configured to form a fuel passage to supply a fuel to a fuel tank; a flap valve configured to open and close a filler port provided in the tank opening-forming member; and a gasket configured to seal between the filler port and the flap valve. The tank opening-forming member has a valve support member configured to form the filler port. The flap valve is supported on the valve support member in a rotatable manner by a shaft support mechanism. In the open-close device for fuel tank, in the process of a fuel supply, the flap valve is pressed by an edge of a refueling nozzle to be rotated via the shaft support mechanism, and the refueling nozzle is inserted through the filler port. In this state, the fuel discharged from the refueling nozzle is supplied through the fuel passage to a fuel tank.
In the open-close device for fuel tank, the gasket is generally designed to exert the highest sealing power when the gasket is bent under application of a force in a direction perpendicular to a plane flush with the gasket (warping direction) from the flap valve. The gasket is, however, likely to receive a force in an axial direction (lateral direction) which is substantially perpendicular to the warping direction, when the flap valve is rotated about a shaft. When receiving a force in the lateral direction from the flap valve, the gasket is deflected in its circumferential direction. This makes it difficult to ensure the high sealing property between the gasket and the flap valve.